(a) Field of the Invention
The present invention relates to a membrane-electrode assembly for a fuel cell, a method of manufacturing the same, and a fuel cell system including the same.
(b) Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol, or natural gas.
Such a fuel cell is a clean energy source that can replace fossil fuels. It includes a stack composed of unit cells, and produces various ranges of power output. Since it has four to ten times higher energy density than a small lithium battery, it has been high-lighted as a small portable power source.
Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell, which uses methanol as fuel.
The polymer electrolyte membrane fuel cell has an advantage of a high energy density, but it also has problems in the need to carefully handle hydrogen gas and the requirement of accessory facilities such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like in order to produce hydrogen as the fuel gas.
On the contrary, a direct oxidation fuel cell has a lower energy density than that of the polymer electrolyte membrane fuel cell, but it has advantages of easy handling of fuel, being capable of operating at room temperature due to its low operation temperature, and no need for additional fuel reforming processors.
In the above fuel cell, the stack that generates electricity substantially includes several to scores of unit cells stacked in multi-layers, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly has an anode (also referred to as a fuel electrode or an oxidation electrode) and a cathode (also referred to as an air electrode or a reduction electrode) attached to each other with an electrolyte membrane between them.
Fuel is supplied to an anode and absorbed in a catalyst thereof, and the fuel is oxidized to produce protons and electrons. The electrons are transferred into a cathode via an external circuit, and the protons are transferred into the cathode through a polymer electrolyte membrane. An oxidant is supplied to the cathode, and the oxidant, protons, and electrons are reacted on a catalyst at the cathode to produce electricity along with water.